Masked plate xerography



A May 1l, 1965 H.. E. CLARK ETAL 3,182,573

MASKED PLATE XEROGRAPHY Filed Aug. 11, 1961 2 sheets-sheet 1 INVENTOR.HAROLD E. CLARK BY ROBERT W. GUNDLACH MORTIMER LEVY @$7 ORNE y May 11,1965 Filed Aug. 11, 1961 H. E. CLARK ETAL MASKED PLATE XEROGRAPHY 2Sheets-Sheet 2 INVEN TOR. HAROLD E. CLARK YROBERT w. GUNDLACH B Mom-WERLEVY WW@ g@ A TTORNEY United States Patent O MASKED PLATE XEROGRAPHYHarold E. Clark, Penfeld, Robert W. Gundlach, Victor, and Mortimer Levy,Rochester, N.Y., assignors to Xerox Corporation, Rochester, N.Y., acorporation of New York Filed Aug. 11, 1961, Ser. No. 139,965 4 Claims.(Cl. 95-1.7)

This application relates to xerography and more particularly to animproved form of xerographic plate and to methods Iand apparatus for usetherewith.

In the most common form of xerography an electrostatic latent image isiformed on a photoconductive insulating layer of xerographic plate byuniformly electrostatically charging the photoconductive insulatinglayer and then selectively discharging the layer through exposure to apattern of light and shadow. The electrostatic latent image .thusIformed can be developed or made visible through the selectiveelectrostatic attraction of pigmented particles .or the like and thedeveloped image may be viewed on the xerographic plate or elsetransterred to a sheet of paper or other suitable support, Various formsof photoconductive insulating layers are known in the art of xerographyand are pertinent to the present invention. These include but are notlimited to amorphous materials such as anthracene or vitreous seleniumand paint-like ilms comprising photo'conductive pigments such as zinc4oxide and an electrically insulating film forming binder. Suchphotoconductive insulating materials may in certain circumstances beemployed as self supporting lfilms but the xerographic plate of commercegenerally employs a photoconductive insulating material on a lsuitablesupport such as a metal plate, a sheet of paper or the like.

Depending upon the character of the support layer the Iplate may takevarious forms including rigid llat plates, ilexible webs, cylinders andthe like. Itis generally characteristic of such plates that the supportlayer is either opaque or of a highly light scattering nature.Accordingly with conventional yplates the steps of charging the plate,exposing the plate to a pattern of light and shadow, and developing theresulting electrostatic latent image are all carried out on that side ofthe plate carrying the photoconductive insulating layer. Xerographicplates are also known, however, in .which the photoconductive insulatinglayer is carried on a supporting layer which comprises a sheet of glassor other transparent material. Such plates .can be processed in theconventional manner except that image exposure may be eiected throughthe transparent support layer rather than directly onto thephotoconductive insulating layer. Such xerographic plates have greatlyenlarged the ilexibility of the xerographic process and have facilitatedlthe creation of new and useful forms of xerographic apparatus. Suchplates, however, suffer from the disadvantage of being sensitive tolight at both surfaces. This characteristic causes serious drawbacks indesigning simple machines for use with such transparently backed platesand makes their use nearly impossible in normal types of procedureswhich are, for obvious reasons, preferably carried out under ambientlight conditions.

|In accordance with the present invention, however, there is provided anovel form or xerographic plate which can vbe charged and developed bycompletely conventional techniques, and on which an electrostatic latentimage can be formed by exposure to a pattern of light and shadow througha transparent support layer but which is substantially insensitive toillumination applied directly to the photoconductive insulating layer.In accordance with the present invention there are also pro- 3,l82,573Patented May 11, 1965 vided novel apparatus and methods employing thenovel Xerographic plates of the present invention.

Fur-ther description of the invention will be made in conjunction withthe drawings in which:

IFIG. 1 is a cross sectional view of a plate according to the invention;

FIG. 2 is a schematic view illustrating one method of making a plateaccording to the invention;

FIG. 3 illustrates the utilization of a plate according to theinvention;

FIG. 4 represents a for-m of copying apparatus according to theinvention; and F IG. 5 represents a modication of the apparatus of FlG.1 illustrates the structure of a xerographic plate 1t) made according tothe present invention. Reference numeral 11 represents a transparentsupport layer for the plate. This layer preferably comprises a sheet ofglass but may also comprise various forms of plastic sheet or othertransparent materials. Inasmuch as layer 11 will generally be anexcellent electric insulator it is desirable, in accordance withconventional Xerographic practice, to provide a transparent electricallyconductive layer 12 on transparent support layer 1-1. When layer 11 isof glass layer 12 will generally be an extremely thin layer of tinoxide. It will be appreciated that layer yi12 as well as the otherlayers in this iigure are shown schematically and .are notrepresentative of the true relative thickness of the various layers.Glass sheets with a transparent conductive tin oxide coating arecommercially available under the trade names Nesa and Electropane Wherelayer `111 is a plastic tin oxide is not generally suitable `for layer12 since its application requires very high temperatures. Accordinglyother conductive coatings such as copper iodide may be employed. Coatedon top of transparent conductive layer 12 is a layer 13 ofphotoconductive insulating material. This may be of the types alreadydescribed or of any other type generally known and employed in the artof xerography. iFinally there is applied over the photoconductiveIinsulating layer 13 a light desensitizing layer 14.

'Plate 10 is shown for illustrative purposes as being mounted in a plateholder 15 equipped with a dark slide 16. Plate holder 15 resembles theplate and lm holders used in photography and is substantially identicalwith the plate holder widely used in commercial xerography except thatdark slide 16 when in position blocks light from reaching support layer11 rather than pho-toconductive insulating layer 13.

Light desensitizing layer 14 may be either a layer which issubstantially opaque to actinic light or else it may be asemi-conductive layer having certain special properties which will bedescribed subsequently. While it would appear simple to devise asuitable opaque overcoating fior the photoconductive insulating layer ofa xerographic plate such has not been the case. The overcoating must notonly be substantially opaque at those wave lengths which impart`conductivity to the photoconductive insulating layer but it must alsobe sutiiciently thin and have suciently excellent dielectric propertiesto permit the Xerographic plate to accept and retain an electrostaticcharge :and also to permit the formation and retention of anelectrostatic `latent ima-ge in response to illumination of the plate.flt has, however, been Ifound possible to meet these requirementsthrough the use of a `layer -14 comprising a suitable dye dissolved in asuitable resinous film forming material.

FIG. 2 represents a form of apparatus suitable for applying an opaqueovercoating to a Xerographic plate. The apparatus includes a container20 lled with a coating liquid 21 and adapted to contain a xerographicplate 10 3i suspended by a string 22 which is wound upon a drum 23 whichin turn is driven at low speed by a motor 24. In operation plate 1t) isimmersed in container 20 and slowly withdrawn therefrom thus forming athin uniform coating on plate liti. A suitable coating liquid for use inthis apparatus comprises three liters of ethylene dichloride in which isdissolved 50 grams of the dye known as nigrosine base 163() and 300grams of cellulose acetate butyrate such as Eastman Kodak CAB-l7l-2. Axerographic plate comprising an approximately 2O micron thick layer ofvitreous selenium deposited upon a tin oxide coated glass plate wasimmersed in and withdrawn from the above described coating liquid at arate of l" per second thereby forming on the plate an overcoating layerapproximately 7 microns thick. By drying and recoating several times itis possible to form a substantially opaque layer having a thickness ofabout 25 microns. This procedure alsoA forms an overcoating layer on thetransparent support layer lil. aswell as on the photoconductiveinsulating layer. The undesired layer` formed on l the support layer canbe readily removed however by wiping with a, cloth saturated withethylene dichloride. rThe particular coating method shown in FG. 2 iswell adapted tothe present invention but other known coating methodssuch as spraying or flow coating may also be used.

Xerographic plates made by the above procedure can readily accept aninitial electrostatic charge of 400 volts and are. substantiallyinsensitive to light directed through layer 14, an exposure of at leastseveral thousand foot candle seconds being required to dissipate thecharge on the plate. The plates are however normally sensitive to lightdirectedthrough transparent support layer 11 and are capable of formingaV developable electrostatic latent image when so exposed to a patternof light and shadow.

When thicker overcoatings such as, for example, coatings greater than2,5 microns in thickness were applied it was found that the plates couldbe electrostatically charged without diiiiculty but that this chargecould not be dissipated by exposure to light at either surface of theplate, thus making such plates useless in xerography.

Light desensitizing layer 14 may also comprise a thin semi-conductive orphotoconductive insulating layer which is substantially opaque toactinic light and which isV capable of conducting electrical chargedcarriers ofl one polarity only. Semi-conductive materials, includingphotoconductors, conduct electricity either in the form ofnegativeelectrons or positive holes The photoconductive insulatingmaterials used in xerography generally conduct by both mechanismsalthough one may predominate. Thus the selenium xerographic layersV incornmon use conduct both holes and electrons although they are betterconductors of holes than of electrons. Methods are also known wherebysuch layers may beV formed which are better electron than holeconductors. When such a layer is charged and illuminated the actiniclight, is absorbed in avery short distance and electron hole pairs areformed near the surface of the layer. Depending upon the polarity of theapplied charge one polarity of charged carrier diiuses towards thesurface of the layer while the other polarity diffuses away from thesurface and it is this latter polarity which4 is effective indischarging the plate. If, however, the surface of the photoconductiveinsulating layer comprises a material which is a poor conductor for thatpolarity of charge carrier which diffuses away-from thesurface the layerwill be effectively insensitive to light. ln terms of a specific examplea conventional vitreous selenium may be formed on a transparent supportlayer andV coated with a thin layer of selenium which contains deepelectron traps and is therefore substantially incapable of conductingelectrons. If such a plate is negatively charged and conventionallyexposed to light the incidentvlight will be absorbed inthe uppermostlayer of selenium and will generate electrons and holes therein. Theholes will tend to Ymove toward the surface of the selenium buty sincethey are already very close to the surface such movement is ineffectivein neutralizing the electrostatic charge on the plate. The electrons areimmediately immobilized in the surface layer and therefore they are alsounable to contribute towards electrically discharging the plate.Accordingly, such a plate, when negatively charged, is substantiallyinsensitive to light when conventionally exposed. If, however, the plateis exposed to the transparent support layer electron hole pairs aregenerated near the interface of the selenium with the transparentsupport layer and the hoes are free to travel across the photoconductiveinsulating layer 13 and thus discharge the plate since ordinary seleniumof high purity is a good hole conductor. Accordingly such a plate issensitive to light when exposed through the transparent support layer.

A plate of the foregoing type may be made by placing a tin oxide coatedglass memberror other suitable transparent support in a vacuum chamberand evaporating high purity selenium onto the glass in accordance withconventional vacuum deposition procedures as are known in connectionwith the xerographic and other arts. Shortly before the conclusion ofthe evaporation process a small amount of oxygen is admitted to thevacuum charnber and becomes incorporated in the surface portions of theselenium layer. It is also possible, and generally preferable, to admitoxygen throughout the evaporation process. Selenium which iscontaminated or doped with oxygen is a very-poor conductor of electrons.Consequently xerographic plates made according to the foregoingprocedure exhibit the desired properties of light sensitivity whenexposed from the back and light Vinsensitivity when exposed from thefront. The above plate making procedure is intended to be illustrativeonly and other materials known to the semi-conductor art may be used inaccordance with the desensitizing principles of the invention.

It is also feasible and within the scope of this invention to form thelight desensitizing layer in a half-tone pattern.

FIG. 3 illustrates one mode of utilization of the xerographic plate ofFIG. l.. FIG. 3A represents or shows the charging or sensitizing of theplate liti; This is accomplished moving the plate relatively to aconventional corona discharge device 3l which is connected to a highvvoltage power supply 32'. This is a completely convenof the plate l()after it has been charged. The plate isV positioned within alight-tightcamera 33, dark slide 16 is removed and the plate is exposedto a suitable original subject 34 by lamps 35 and lens 36 mounted asshown in a light-tight partition 37. Dark slide 16 is then replaced andthe plate may be removed from the camera. The exposure step is the onlyone which cannot be carried out in normal room light. The electrostaticlatent image formed in the exposure step is then developed or madevisible as shown in FlG. 3C. The plate lil-is positioned at an obliqueangle from the horizontal with its lowest end resting in a tray 38. Asupply of cascade developer material 39' is then poured over the uppersurface of the plate ltl, i.e., the surface opposite that of thetransparent support layer. As ,is well known in the xerographic artcascade developer materials generally comprise a mixture of micron sizepigmented toner particles mixed with much larger carrier beads. Suchmaterials are commercially available from Xerox Corporation, Rochester,New York. When developer 39 is poured over plate 10 some of the tonerparticles are selectively attracted Vto plate 1t) by the electrostaticlatent image thereon and form a visible pattern corresponding to theoriginal subject matter 34 of FIG. 3B; Various xerographic developermethods and materials are known besides the one just described and maybe used in conjunction with thev presentrinvention. The describedmethod, however, is a particularly simple and effective procedure whichhas not heretofore been practical because it was necessary that theprocedure be carried out in a dark room or the like. In accordance withthe present invention, however, this simplied development method attainspracticality because it can be performed in ordinary room light andrequires no dark room, dark box or other equipment other than the simpleelements shown in FIG. 3C.

The image on plate may then be transferred to a sheet of paper 40 or thelike as shown in FIG. 3D. Paper 40 is laid over the image on plate 10which is then passed beneath a corona discharge device 31 which may bethe same device as shown in FIG. 3A. When paper 40 is removed from plate1t) it is found to carry the toner image with it. The transfer procedureof FIG. 3E may also be used following that of FIG. 3D or as a substitutetherefore. In accordance with FIG. 3E paper 4t) is first removed fromplate 1) in connection with a roller 41 which is rolled across paper 40.Thus any given portion of paper 40 is in contact with roller 41 as itleaves plate 1t) and is not separated from roller 41 until after it hasbeen separated from plate 10. Roller 41 is at least slightlyelectrically conductive and is connected to a power supply 42 whichsupplies a voltage generally in the range of 500 to 900 volts althoughthe voltage may also be zero. When this transfer procedure is followedthe developed image adheres to and is transferred to paper 4t) while theelectrostatic latent image on plate lll is not destroyed and remainssubstantially as it was prior to development. Accordingly the plate mayagain be developed as shown in FIG. 3C and the newly developed image mayagain be transferred as shown in FIGS. 3D and 3E or FIG. 3E alone. Thisprocedure, which is more fully described in U.S. Patent 2,951,443 may berepeated many times to form many copies of an original subject with onlyone charginv of plate 10 and only one exposure of the plate to theoriginal subject. While the basic procedure is known and is fullydescribed in the above referenced patent it could not previously becarried out in the manner shown herein because all the steps had to becarried out in darkness or at least under safe light illumination. Themulticopy procedure of the patent avoids destroying the electrostaticlatent image in the transfer step but does not prevent destruction ofthe image if the plate is permitted to be exposed to room light. Inaccordance with the present invention, however, a multicopy process maybe carried out as illustrated in normal room light and accordinglybecomes feasible for use in small oi'lices and the like which have onlysimple manual xerographic equipment where transfer must be carried outmanually and in an exposed location.

FIG. 4 is a schematic representation of a simple and novel xerographiccopying apparatus particularly adapted for use with the xerographicplates of the present invention. The apparatus includes a cubical box ora similar light-tight enclosure 50 which is mounted in brackets 51 forrotation about a horizontal axis. One of the faces of the box parallelto the axis of rotation incorporates a hinged copy board 52 asillustrated. A pair of fluorescent lamps 53 is positioned within box Siland adapted to illuminate copy board 52. Also positioned within box 5t)are a reflecting prism 54 and lens 55 which are adapted to project animage of any material on copy board 52 onto xerographic plate 1li, whichis mounted in one of the faces of box Sil which is parallel to the axisof rotation and also adjacent to the face carrying copy board 52. Plate1i) may be the same structure as shown in FIG. l and is mounted with itstransparent support layer facing into the box. Plate holder 15,illustrated in FIG. l, is not required with this form of apparatus andis preferably omitted. Positioned above plate 10 and mounted on hinges56 is a charging grid 5'7 which is connected to a high voltage powersupply 32. Charging grid 57, which may be of the known type described inU.S. Patent 2,932,742 comprises a grid of tine wires covering the entireoperative area of plate 10 and is adapted to uniformly electrostaticallycharge plate 10 without movement relative thereto. The face of box 50which carries plate 10 is also provided at its outer edge with a set ofprojecting lips 58 which are adapted to retain a quantity of xerographicdeveloper 39. In operation a sheet of printed matter or other originalsubject to be copied is placed on copy board 52 which is then swung upuntil it is ush with box 50, the box meanwhile being rotated to keep thecopy board in an approximately horizontal position, unless clips or thelike, not illustrated, are provided to keen the original subject fromsliding on the copy board. Power supply 32 is then brieily energized toput an electrostatic charge on plate 1l) and lamps 53 are next brieflyenergized to project a light image of the original subject on copy board52 onto plate 10. Charging grid 5'7 is then swung away from plate 10 andbox 50 is slowly rocked back and forth on its axis about the position inwhich plate 10 is uppermost, thereby causing developer 39 to flow backand forth over plate 1t) and form a developed image thereon. Thedeveloped image may be transferred to a sheet of paper or the like byplacing the paper on plate 10 and repositioning and reenergizingcharging grid 57, after which paper may be removed bearing a rightreading reproduction of the original subject matter. Transfer may alsobe carried out according to the procedure described in connection withFIG. 3E, in which case box 50 may again be rocked to develop the imageon plate 10 and in this manner numerous copies may be made of theoriginal subject without recharging or reexposing plate 1i). It will benoted that this apparatus functions in ordinary room light and thereforeonly becomes practical in conjunction with xerographic plates which areinsensitive to light at the surface on which the image is developed.

FIG. 5 shows a portion of the apparatus of FIG. 4 incorporating certainmodifications. Plate 10 is mounted on a set of arms 58 and may be movedfrom the illustrated position to the one indicated by dotted lines byoperating handle 59 which is preferably positioned on the outside of boxSt). A corona charging device 31 of conventional design is connected tohigh voltage power supply 32 and is positioned above the illustratedposition of plate lil but below the alternate position thereof. Chargingdevice 31 slides upon guide bars 60 and is mechanically attached to arod 61 which extends through the wall of box 50 and permits the chargingdevice to be moved from outside the box. Bars 6) and rod 61 are sopositioned that when the charging device 31 is withdrawn to one sideplate 10 can be moved from the illustrated position to the alternateposition in which it is sealed against the side of box 50 by rubbergaskets S2. Positioned above plate 10 on the outside of box 50 is a pairof guide bars 53 on which is slidably mounted an electrically conductiveresilient roller 41 connected to a power supply 42. Roller 41 is adaptedto be manually pressed in engagement with plate 10 by a handle 64. Inoperating this machine the original copy is inserted as in FIG. 5, powersupply 32 is energized and rod 61 is operated to move charging device 31across plate 10, after which power supply 32 is turned off and handle 59operated to move plate 1t) against gaskets 62. Development is thencarried out exactly as described in connection with FIG. 4. Transfer iscarried out by laying a piece of paper or the like against plate 10,energizing power supply 42 to apply a voltage of from 500 to 900 voltsto roller 41, and rolling the roller across the paper with the aid ofhandle 64, while simultaneously peeling the paper away from plate 10 asshown in FIG. 3E. Since the transfer procedure does not destroy theelectrostatic latent image on plate 10, many copies may be made in rapidsuccession by single exposure by the procedures already described. Itwill be noted that in this form of apparatus the charging equipment ishoused within box Si), thereby allowing greater freedom in choosing andoperating the transfer apparatus, which may accordingly be quitedistinct from the charging apparatus as illustrated, for example, inthis gure.

The improved plates and procedures of the present invention are alsoadapted to the recording of X-ray images or other images of penetratingradiation as well as to the recording of visible light images. It iswell known that Xerographic plates are sensitive to X-rays, gamma rays,beta rays and like forms of high energy penetrating radiation as well asto low energy non-penetrating radiation such as visible or ultra-violetlight. However, this X-ray sensitivity has heretofore been coupled witha sensitivity to visible light, and it has accordingly been necessary tokeep the photoconductive insulating layer of an X-ray recording plateshielded from visible light from a time prior to charging to a timesubsequent to development. This considerably complicates the making ofxerographic X-ray images because the same photoconductive insulatinglayer must be rn'ade accessible for charging and developrnent. Platesaccording to the present invention, however, are insensitive to lightimpinging on the exposed surface of the photoconductive insulatinglayer, and it is thus necessary only to prevent visible light fromreaching the opposite side of the photoconductive layer in order torender the plate insensitive to visible light While still remainingsensitive to X-rays. This can readily be accomplished by leaving darkslide 16 of FIG. 1 in position Vat all times or by coating support layer11 with black paint or other opaque material or by making support layer11 of a material such as aluminum which is opaque to visible light whilepreferably transparent to X-rays. Such a plate may be charged, exposedto a pattern of X-rays and developed in normal room light and withoutany special apparatus or manipulations being required to protect theplate from ambient light. Exposure to the X-ray pattern may be made tothe front of the plate, or where support layer 1l is suliicientlytransparent to penetrating radiation it may also be made to the back ofthe plate. The present invention thus provides for X-ray Xerography thesame advantages and simplifications as it provides for visible lightXerography. In addition it minimizes the undesirable effect known asfatigue which is otherwise caused by excessive exposure of an X-ray orother Xerographic plate to ambient light.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in specific embodiments thereof, it isnot intended to be limited thereby, but it is intended to cover theinvention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A Xerographic apparatus comprising in combination a light-tighthousing, an aperture in said housing, a xerographic plate positionedrelatively to said aperture whereby only the inner surface of said plateis adapted to be shielded from ambient light by said housing, the outersurface of said plate being retentive of electric charge and insensitiveto light and the inner surface'being sensitive to light, meanspositioned within said housing to direct a pattern of light and shadowin image configuration on the inner surface of said plate, and meanspositioned on said housing and external thereto to develop anelectrostatic latent image on the outer surface of said plate.

2. Apparatus according to claim 1 further including means positionedwithin said housing to electrostatically charge said plate and imagetransfer means positioned on said housing on the outside thereof.

3. Xerographic copying apparatus comprising a lighttight housing,aperture in said housing, a xerographic plate having a light insensitiveinner surface and a light sensitive charge retaining outer surface,plate positioning means within said housing whereby only the innersurface of said plate is adapted to be shielded from ambient light,moving means for moving said plate positioning means whereby the platecan be moved from a first position lying within said housing to a secondposition wherein said plate is outwardly urged against said aperture andseated thereagainst, charging means positioned within said housing andadapted to apply an electrostatic charge to the outer surface of saidplate while in said first position, exposure means within said housingadapted to project a pattern of light and shadow upon the inner surfaceof said plate, development means mounted external of said housing andadapted to develop an electrostatic latent image on the outer surface ofsaid plate while in said second position, and means mounted external tosaid housing for transferring a developed image from the outer surfaceof said plate without destroying the electrostatic latent image thereon.

4. Xerographic apparatus according to claim 3 in which the light-tighthousing is mounted for rotation about a horizontalaxis and in whichimage development is effected by such rotation. Y

References Cited by the Examiner UNITED STATES PATENTS 2,297,691 10/42Carlson 95-1.7 X 2,550,738 5/51 Walkup.

2,551,582 5/51 Carlson 95-l.7 X 2,588,675 3/52 Walkup et al 95-1.72,624,652 1/53 Carlson 95-1.7 X 2,684,901 7/54 Sabel et al. 95-1,7 X2,689,179 9/54 Walkup et al 95-1.7 2,693,416 1 1/ 54 Butterfield.

2,758,525 8/56 Moncrieff-Yeats 95-1.7 X 2,791,949 5/57 Simmons et al951.7 2,831,409 4/58 Bixby et al. 95-1.7 2,951,443 9/60 Byrne 1014262,996,400 8/61 Rudd et al 95-1.7 X

EVON C. BLUNK, Primary Examiner.

OSEPH P. STRIZAK, Examiner.

1. A XEROGRAPHIC APPARATUS COMPRISING IN COMBINATION A LIGHT-TIGHTHOUSING, AN APPERTURE IN SAID HOUSING, A XEROGRAPHIC PLATE POSITIONEDRELATIVELY TO SAID APERTURE WHEREBY ONLY THE INNER SURFACE OF SAID PLATEIS ADAPTED TO BE SHIELDED FROM AMBIENT LIGHT BY SAID HOUSING, THE OUTERSURFACE OF SAID PLATE BEING RETENTIVE OF ELECTRIC CHANGE AND INSENSITIVETO LIGHT AND THE INNER SURFACE BEING SENSITIVE TO LIGHT, MEANSPOSITIONED WIHTIN SAID HOUSING TO DIRECT A PATTERN OF LIGHT AND SHADOWIN IMAGE CONFIGURATION ON THE INNER SURFACE OF SAID PLATE, AND MEANSPOSITIONED ON SAID HOUSING AND EXTERNAL THERETO TO DEVELOP ANDELECTROSTATIC LATENT IMAGE ON THE OUTER SURFACE OF SAID PLATE.